The present invention relates to a continuously variable transmission comprising a hydraulic pump and a hydraulic motor.
With a continuously variable speed transmission comprising a closed circuit disposed between a hydraulic pump and a hydraulic motor at least one of which is of variable displacement type, as disclosed in the Japanese Patent laid-open Publication No. 56(1981)-95722, a bypass line is provided between the pump and motor to communicate two hydraulic lines which constitute the closed circuit, and the transmission is clutch controlled by opening and closing the bypass line with a opening control valve (clutch valve).
In a vehicle having such a continuously variable speed transmission mounted thereon, when the opening of the accelerator pedal is brought to zero with the operator's foot released, such control has been adopted as to prevent engine stall by opening the clutch valve when vehicle speed falls below a given value.
In such control as described above, to prevent engine stall for instance, even when quick braking is applied in the minimum speed reduction ratio (top condition), it is necessary to set somewhat higher the vehicle speed at which the clutch valve starts to open.
The high vehicle speed for starting the clutch actuation may be desirable in preventing engine stall at such a case as described above particularly for a quick stop. However, in a case where the vehicle is decelerated slowly, when the accelerator is depressed while the vehicle is still running, the engine speed is increased abruptly, thus resulting in an uncomfortable running condition. This takes place for the following reason: even with the clutch valve having been opened, in the running vehicle, the hydraulic motor in a continuously variable speed transmission is rotating corresponding to the wheel rotation and is drawing the delivered hydraulic oil from the hydraulic pump, thus reducing the engine load required to drive the pump. Therefore, the engine speed tends to be increased rapidly in response to the depression of the accelerator pedal.